I am writing a user manual and am stuck upon these include and this includes. Do I use This includes when there is only one thing involved? For example, "This includes normal users" and "This includes the front door". And do I use These include when there are two or more things? For example: "These include the lights and doors", "These include administrators and normal users".
As Sam said in his comment, the crux is what precedes the "this/these include(s)", not what follows. As a matter of fact, "This includes administrators and normal users" can be just fine, depending on the context.
That said, these can still be a little tricky. For example:
The "these include" refers to "several different users." But:
In this case, the "this includes" is singular, because it refers to the single principle that no user is allowed to change a password.
Here, "these include" refers back to the many levels where corruption was found.
However, instead of trying to figure out the correct usage, a better option would be to use the more all-encompassing including. In fact, this change seems to improve the sentences:
Get rid of this this, and this problem goes away.
A footnote (to underscore how tricky the this/these choice can be): I could have said, "In fact, these changes seem to improve the sentences..."
Which is correct? If I am referring to the single hint of replacing the phrase "this includes" with the word including, then "this change seems to improve" is appropriate. But, if I'm referring to the three individual improvements found in each example, then "these changes" would be better.
then surely use
Check here for particular cases of use.